The present invention is concerned with thermosettable, polymerizable resin compositions comprising polyepoxides and polyisocyanates, and more particularly with highly stabilized thermosettable, polymerizable resin compositions. Because of the stabilized characteristics of the composition, it has a remarkably prolonged shelf life or storage life around room temperature even in the presence of a basic curing catalyst for promoting the curing reaction of the composition. Further, the composition is highly stabilized so that the composition can be mixed with various additives such as filler powders, or so that the composition can be impregnated into fibrous materials such as glass cloth at slightly elevated temperatures without premature gelation of the composition.
Thermosettable, polymerizable compositions comprising polyepoxides and polyisocyanate are known as disclosed in German Offenlegungsschrift No. 2,359,386 and a U.S. Pat. No. 3,494,888. These compositions can produce cured products having excellent mechanical and electrical properties at high temperatures. The products exhibit excellent thermal stability or thermal resistance. For these reasons, the compositions are becoming used widely in the fields of, for instance, impregnation, casting and molding of electronic and electric devices.
The resin compositions are polymerized and cured by heating in the presence of a basic catalyst such as morpholine, imidazole or the like as disclosed in the German Offenlegungsschrift No. 2,359,386. The catalyst promotes the formation of oxazolidone rings formed by epoxy groups and isocyanate groups and of isocyanurate rings or uretdione rings formed by isocyanate groups.
When the composition contains the basic curing catalyst, it easily gels at low temperatures such as room temperature within a short period of time. Even if the composition contains no basic catalyst, it was found that insoluble substances precipitated from the composition within about 3 months at room temperature. Although the mechanism of the precipitation has not been elucidated, it is speculated that basic substances contained as impurities in the polyepoxide and polyisocyanate may have some influences on the precipitation.
On the other hand, when the composition containing no basic catalyst is heated to a slightly elevated temperature such as 50.degree. C. so as to blend the components or to admix the components with additives such as fillers, the composition easily gels within about 2 to 3 weeks. The instability of the composition is a serious disadvantage when a large quantity of liquid composition is kept at slightly elevated temperatures in a tank so as to repeatedly use it for impregnation. The shortage of the stability of the composition free of the basic curing catalyst may be caused by the basic impurities. The instability is also observed when the liquid composition free of the basic curing catalyst is impregnated into fibrous insulatig materials to which the basic curing catalyst is added prior to impregnation.
U.S. Pat. No. 3,494,888 discloses thermosettable, polymerizable compositions comprising polyepoxide monomers or polymers and polyisocyanate polymers terminated by isocyanate groups and containing an isocyanurate ring or uretdione ring, or both. In preparing the polyisocyanate polymers, monomeric polyisocyanates are trimerized or dimerized in the presence of a basic catalyst. In order to obtain a desired polyisocyanate polymer terminated by free isocyanate groups, the trimerization or dimerization of the polyisocyanate is stopped by acidic substances such as acetic acid anhydride, toluenesulfonate, hydrogen chloride, phosphoric acid, chloroacetic acid, carboxylate chloride, carboxylic acids, mineral acids, etc.
According to the experimentation with respect to effects of the acidic substances disclosed in this U.S. patent on the stabilization of the composition, it was found that the acidic substances were not effective for the stabilization, while they may be useful as stoppers for the basic polymerization catalysts used in this U.S. patent.